The present invention is concerned with a novel process for the manufacture of the optically active lactone of the formula ##STR2## wherein R represents the benzyl residue.
This optically active lactone of formula I is a known, valuable intermediate in the synthesis of (+)-biotin, as well as of derivatives thereof and compounds related thereto.
Under the expression "(3aS,6aR)" in connection with formula I there is to be understood in the scope of the present invention that antipode which is dextrorotatory in benzene or chloroform. This antipode is referred to hereinafter as the (+)-lactone.
Processes for the manufacture of the (+)-lactone of formula I are already known from German Patent Specification No. 2058248 (corresponding to U.S. Pat. No. 3,700,659) and from European Patent Publication No. 44 158 (corresponding to U.S. Pat. No. 4,403,096). In the first case, racemic half esters are resolved into their optical antipodes and the desired antipode is converted into the (+)-lactone of formula I. In the second case, a dicarboxylic acid or the corresponding anhydride is reacted with a particular optically active amine, whereby the (S)-amic acid is formed in excess. This can subsequently, after esterification of the carboxyl group, be reduced with sodium borohydride and hydrolyzed to the desired (30 )-lactone of formula I. Both processes have disadvantages, in that on the one hand a racemate resolution must be carried out with recyclization of the undesired antipode and on the other hand in order to obtain a good yield the likewise formed (R)-amic acid as well as other byproducts must also be recyclized. Moreover, in the latter case the carboxyl group which is still free after the amide formation must be esterified in order to permit the required reduction to the (+)-lactone of formula I.